Spatial Light Modulators (SLMs) are widely used in the industry for video monitors, graphic displays, projectors, and hard copy printers. Spatial light modulators typically modulate incident light to form a light image. This light image is directed to a screen in the case of a projector, video monitor or display, or is ultimately focused on a light sensitive material, such as a photoreceptor drum, in the case of a xerographic printer.
A recent innovation of Texas Instruments Incorporated of Dallas, Tex. is the digital micromirror device (DMD). The DMD is a spatial light modulator suitable for use in displays, projectors and hard copy printers. The DMD is a micromechanical monolithic single-chip integrated circuit, comprised of a high density area array of 16 micron square deflectable micromirrors on 17 micron centers. These mirrors are fabricated over address circuitry including an array of memory cells and address electrodes, these memory cells being controlled by a row address circuit and loaded/unloaded by column pixel data shift registers. Each mirror forms one pixel of the DMD array, and is bistable through electrostatic attraction forces, that is to say, stable in one of two deflected positions. A source of light is directed upon the mirror array, and is reflected in one of two directions by each mirror. In one stable "on" mirror position, incident light is reflected by the mirror to a collector lens and focused on the display screen, or directed to the photoreceptor drum, and comprises one pixel. In the other "off" mirror position, light directed on the mirror is deflected to a light absorber. Each mirror of the array is individually controlled to either direct incident light into the collector lens, or, to the light absorber. The collector lens may be used in combination with a light prism to ultimately focus and magnify the light image from the pixel mirrors when projected onto a display screen to produce a viewable image. If each pixel mirror of the DMD array is in the "on" position, the light image will be a bright array of pixels.
For a more detailed discussion of the DMD device, cross reference is made to U.S. Pat. No. 5,061,049 to Hornbeck, entitled "Spatial Light Modulator and Method"; U.S. Pat. No. 5,079,544 to DeMond, et al, entitled "Standard Independent Digitized Video System"; and U.S. Pat. No. 5,105,369 to Nelson, entitled "Printing System Exposure Module Alignment Method and Apparatus of Manufacture", each patent being assigned to the same assignee of the present invention, and the teachings of each are incorporated herein by reference. Gray scale of the pixels forming the image is achieved by pulse width modulation techniques of the mirrors, such as that described in U.S. Pat. No. 5,278,652, entitled "DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System", assigned to the same assignee of the present invention, and the teachings of each are incorporated herein by reference.
The DMD device utilized in printing systems is an exceptionally elongated array of micromirrors, typically having a length in excess of 5 inches. Maintaining the optic's ability to focus on the full length of the DMD chip without compromise requires that the DMD integrated circuit be kept flat to within 1-3 mils. Therefore, the packaging can not induce warp or stress on the DMD device, i.e. package lid needs to be sealed to the package base without inducing warp or stress on the device. It is also critical that the package be sealed to prevent permeation of the ambient into the package, such as moisture and other impurities which can degrade or inhibit the operation of the tiny micromirrors and cause stiction, i.e. the mirrors sticking on the landing electrodes.
The DMD is a special micromechanical device requiring unique packaging elements, such as an optic window, in the lid. The package is difficult to assemble using more conventional IC packaging techniques. For instance, thermally cured adhesives induce warp in the device due to thermal mismatch of the package parts. Hermetic, seam welded packages are extremely difficult to fabricate due to the extensive length of the device and can also induce warp.
It is desired to provide a package that is suitable for encasing integrated circuits that need to be maintained flat, especially elongated devices such as the micromechanical DMD devices used in digital printing systems. The package should also be easy to disassemble to allow changing of the glass or the DMD device in the package. The package should maintain flatness of the integrated circuit device housed therewithin, and keep moisture out of the package to avoid degradation of the integrated circuit operation. In addition, the package should allow sealing of the package without subjecting the integrated circuit to elevated temperatures, to avoid damage to the micromechanical device.